Monthly useage when transferring from Unlimited to fibre

[#1]

I just wondered what will happen to my monthly allowance today when I switch from unlimited to fibre which has a 120Gb limit. I checked my account earlier and to my shock, I found that I have already been assessed as using 30GB of the 120GB for this month.
As this is an 18 month contract, I was expecting to get a refund on my unlimited payment, pay a months payment for the new fibre product and start with a full month's allowance.  I have raised a ticket with Plusnet but wondered what I can expect & if any of you guys had the same happen to them?
Katie

[#2]

I just realised that my useage only goes to the 18th March and is not for a  full month so I disn't really have a quibble about it. I have asked Plusnet to ignore the ticket. I should have woken up and thought it out before I posted. 
Katie

[#3]

I would expect your change of account type to occur on your billing date - that is the normal procedure.

[#4]

We ordered FTTC a couple of days ago and there is a closed ticket on our account which confirms that we have an account type change scheduled for the morning of the engineer appointment.
Alex

[#5]

Thanks for the info jelv. It does seem that the new product  will go from the next billing date. I hadn't realised that.
Alex - we got that closed ticket too and I have had an email today confitming that the fibre product is now current. I'm just awaiting the BT guy now. The letter which came with the modem said I'd get a call from the engineer when they start work on the line & then when he is coming to the house.
Katie